The present invention is concerned with a probe for use in the extraction of analytes from a sample.
There are many instances where it is necessary to analyse for components within a liquid, for example in the measurement of water and food quality. The method widely used in these applications include the use of gas chromatography or gas chromatography interfaced with mass spectrometry. However, before these components can be analysed by gas chromatography, they must be present in the gas phase.
A number of techniques are available to extract organic material from the liquid phase into the gas phase.
These include equilibrium headspace, purge and trap and solid phase extraction (SPE)
Equilibrium headspace is a technique where the components in the liquid phase are allowed to partition into the gas phase above the surface of the sample. After a period during which the partition into the gas phase may be enhanced by heating and agitation of the liquid, an equilibrium is established between the components in the liquid and the gas phase. The gas phase is then transferred to the analyser for analysis.
Unlike Equilibrium Headspace, in purge and trap, the gas above the liquid is continuously replaced with new gas. As a result, no equilibrium is formed and eventually virtually all the components are extracted from the liquid. As large volumes of purge gas may be required to extract all the components from the liquid, an adsorbent trap is typically used to reduce the sample volume before GC.
While Headspace and purge and trap are suited for components with a high solubility in the gas they are less efficient for components with low vapour pressure or of high soluability in the liquid. In these cases SPE may be more appropriate. In this case the components are extracted from the liquid using a solid (phase) absorbent and subsequently recovered into the gas phase using thermal desorption.
However it has been difficult to combine automation with the most efficient extraction.
One attempt to automate the process is termed Solid Phase Micro Extraction (SPME). In this approach the absorbent used to extract the component from the liquid, is coated on a fibre as a thin layer. The fibre is immersed in the sample for a time and then passed directly to the GC where it is thermally desorbed. As the fibres replace the conventional needles, the process is compatible with normal liquid autosampler that are widely available for GC. In this way there may be some degree of automation
The disadvantage with SPME is that only a limited amount of adsorbent can be loaded onto a fibre. Since the process requires equilibrium between the two phases, the extraction efficiency depends on the mass of solid phase. As a result the SPME suffers from a low capacity for the components.
In an attempt to address this, the solid phase has been coated onto rods placed in the sample. The increased size of the rod permits a higher degree of coating and with it improved extraction efficiency. To increase the efficiency further such solid phase may be coated onto a magnetic stirring element. These are made to rotate with a magnetic stirrer. Further extraction efficiency may be achieved through sonic agitation. Alternatively the solid phase may be coated onto a non-magnetic stirring element with the liquid stirred using some other means. As with fibres, the rods are then thermally desorbed to release the components to the GC.
However such rods are not readily compatible with conventional autosampler. The current practice is to manually, in turn, remove the rod, wash with suitable wash solution (usually water) and dry using paper. The rods are then manually loaded into a desorption tube
It is therefore an aim of the present invention to alleviate at least some of the disadvantages identified above.
It is a further aim of the present invention to provide a probe for use in extracting analytes from a sample.
It is a further aim to provide a probe which is suitable for use with an automated process.